1. Field of the Invention
The present invention relates to a control system for an internal combustion engine, which is configured to be capable of changing an intake air amount by changing operating characteristics of an intake valve, using a first variable valve-actuating mechanism and a second variable valve-actuating mechanism having a lower response speed than that of the first variable valve-actuating mechanism.
2. Description of the Related Art
Conventionally, as a control system for an internal combustion engine of this kind, one disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2006-57573 is known. This combustion engine is provided with a first variable valve-actuating mechanism for changing the valve lift of an intake valve, and a second variable valve-actuating mechanism for changing the central angle of an operating angle of the intake valve (hereinafter simply referred to as “the central angle”). The first and second variable valve-actuating mechanisms use an electric motor and an oil pressure pump as drive sources thereof, respectively, and the response speed of the second variable valve-actuating mechanism, that is, the response speed of the operation amount of the second variable valve-actuating mechanism with respect to a control input therefor is lower than that of the operation amount of the first variable valve-actuating mechanism. In the above-described conventional control system, the intake air amount is controlled by controlling the valve lift and the central angle by the first and second variable valve-actuating mechanisms as follows:
A target central angle, which is a target value of the above-described central angle, is determined by searching a target central angle map according to the load on the engine obtained e.g. by a sensor, and an actual central angle is estimated as an actual central angle equivalent value. In the target central angle map, the target central angle is set to a value which makes it possible to obtain excellent fuel economy of the engine. Further, a target valve lift, which is a target value of the above-described valve lift, is calculated based on the load on the engine and the estimated actual central angle equivalent value. Then, a control input based on the calculated target valve lift is input to the first variable valve-actuating mechanism, and a control input based on the calculated target central angle is input to the second variable valve-actuating mechanism, whereby the valve lift and the central angle are controlled to the target valve lift and the target central angle, respectively. Thus, the response delay of the operation amount of the second variable valve-actuating mechanism with respect to the control input is compensated for, to thereby accurately control the intake air amount.
As described above, in the conventional control system, the target central angle, which is set in the target central angle map to such a value as will make it possible to obtain excellent fuel economy, is used only as the target value of the central angle. As a result, in the conventional control system, when the load on the engine is suddenly increased due to demand of acceleration, the response delay of the second variable valve-actuating mechanism cannot be sufficiently compensated for, which makes it impossible to obtain a sufficient intake air amount. This makes it impossible to increase the output of the engine with high responsiveness to the load on the engine. Further, to eliminate the above inconveniences, it is considered that the target central angle is set in the target central angle map with priority given to the output but not to the fuel economy. In this case, however, it is impossible to obtain excellent fuel economy of the engine.